NAME

SYNOPSIS

DESCRIPTION

FreeBSD provides some packet routing facilities. The kernel maintains a
routing information database, which is used in selecting the appropriate
network interface when transmitting packets.
A user process (or possibly multiple co-operating processes) maintains
this database by sending messages over a special kind of socket. This
supplants fixed size ioctl(2)'s used in earlier releases. Routing table
changes may only be carried out by the super user.
The operating system may spontaneously emit routing messages in response
to external events, such as receipt of a re-direct, or failure to locate
a suitable route for a request. The message types are described in
greater detail below.
Routing database entries come in two flavors: for a specific host, or for
all hosts on a generic subnetwork (as specified by a bit mask and value
under the mask. The effect of wildcard or default route may be achieved
by using a mask of all zeros, and there may be hierarchical routes.
When the system is booted and addresses are assigned to the network
interfaces, each protocol family installs a routing table entry for each
interface when it is ready for traffic. Normally the protocol specifies
the route through each interface as a ``direct'' connection to the
destination host or network. If the route is direct, the transport layer
of a protocol family usually requests the packet be sent to the same host
specified in the packet. Otherwise, the interface is requested to
address the packet to the gateway listed in the routing entry (i.e., the
packet is forwarded).
When routing a packet, the kernel will attempt to find the most specific
route matching the destination. (If there are two different mask and
value-under-the-mask pairs that match, the more specific is the one with
more bits in the mask. A route to a host is regarded as being supplied
with a mask of as many ones as there are bits in the destination). If no
entry is found, the destination is declared to be unreachable, and a
routing-miss message is generated if there are any listeners on the
routing control socket described below.
A wildcard routing entry is specified with a zero destination address
value, and a mask of all zeroes. Wildcard routes will be used when the
system fails to find other routes matching the destination. The
combination of wildcard routes and routing redirects can provide an
economical mechanism for routing traffic.
One opens the channel for passing routing control messages by using the
socket call shown in the synopsis above:
The family parameter may be AF_UNSPEC which will provide routing
information for all address families, or can be restricted to a specific
address family by specifying which one is desired. There can be more
than one routing socket open per system.
Messages are formed by a header followed by a small number of sockaddrs
(now variable length particularly in the ISO case), interpreted by
position, and delimited by the new length entry in the sockaddr. An
example of a message with four addresses might be an ISO redirect:
Destination, Netmask, Gateway, and Author of the redirect. The
interpretation of which address are present is given by a bit mask within
the header, and the sequence is least significant to most significant bit
within the vector.
Any messages sent to the kernel are returned, and copies are sent to all
interested listeners. The kernel will provide the process ID for the
sender, and the sender may use an additional sequence field to
distinguish between outstanding messages. However, message replies may
be lost when kernel buffers are exhausted.
The kernel may reject certain messages, and will indicate this by filling
in the rtm_errno field. The routing code returns EEXIST if requested to
duplicate an existing entry, ESRCH if requested to delete a non-existent
entry, or ENOBUFS if insufficient resources were available to install a
new route. In the current implementation, all routing processes run
locally, and the values for rtm_errno are available through the normal
errno mechanism, even if the routing reply message is lost.
A process may avoid the expense of reading replies to its own messages by
issuing a setsockopt(2) call indicating that the SO_USELOOPBACK option at
the SOL_SOCKET level is to be turned off. A process may ignore all
messages from the routing socket by doing a shutdown(2) system call for
further input.
If a route is in use when it is deleted, the routing entry will be marked
down and removed from the routing table, but the resources associated
with it will not be reclaimed until all references to it are released.
User processes can obtain information about the routing entry to a
specific destination by using a RTM_GET message, or by calling sysctl(3).
Messages include:
#define RTM_ADD 0x1 /* Add Route */
#define RTM_DELETE 0x2 /* Delete Route */
#define RTM_CHANGE 0x3 /* Change Metrics, Flags, or Gateway */
#define RTM_GET 0x4 /* Report Information */
#define RTM_LOSING 0x5 /* Kernel Suspects Partitioning */
#define RTM_REDIRECT 0x6 /* Told to use different route */
#define RTM_MISS 0x7 /* Lookup failed on this address */
#define RTM_LOCK 0x8 /* fix specified metrics */
#define RTM_OLDADD 0x9 /* caused by SIOCADDRT */
#define RTM_OLDDEL 0xa /* caused by SIOCDELRT */
#define RTM_RESOLVE 0xb /* request to resolve dst to LL addr - unused */
#define RTM_NEWADDR 0xc /* address being added to iface */
#define RTM_DELADDR 0xd /* address being removed from iface */
#define RTM_IFINFO 0xe /* iface going up/down etc. */
#define RTM_NEWMADDR 0xf /* mcast group membership being added to if */
#define RTM_DELMADDR 0x10 /* mcast group membership being deleted */
#define RTM_IFANNOUNCE 0x11 /* iface arrival/departure */
#define RTM_IEEE80211 0x12 /* IEEE80211 wireless event */
A message header consists of one of the following:
struct rt_msghdr {
u_short rtm_msglen; /* to skip over non-understood messages */
u_char rtm_version; /* future binary compatibility */
u_char rtm_type; /* message type */
u_short rtm_index; /* index for associated ifp */
int rtm_flags; /* flags, incl. kern & message, e.g. DONE */
int rtm_addrs; /* bitmask identifying sockaddrs in msg */
pid_t rtm_pid; /* identify sender */
int rtm_seq; /* for sender to identify action */
int rtm_errno; /* why failed */
int rtm_fmask; /* bitmask used in RTM_CHANGE message */
u_long rtm_inits; /* which metrics we are initializing */
struct rt_metrics rtm_rmx; /* metrics themselves */
};
struct if_msghdr {
u_short ifm_msglen; /* to skip over non-understood messages */
u_char ifm_version; /* future binary compatibility */
u_char ifm_type; /* message type */
int ifm_addrs; /* like rtm_addrs */
int ifm_flags; /* value of if_flags */
u_short ifm_index; /* index for associated ifp */
struct if_data ifm_data; /* statistics and other data about if */
};
struct ifa_msghdr {
u_short ifam_msglen; /* to skip over non-understood messages */
u_char ifam_version; /* future binary compatibility */
u_char ifam_type; /* message type */
int ifam_addrs; /* like rtm_addrs */
int ifam_flags; /* value of ifa_flags */
u_short ifam_index; /* index for associated ifp */
int ifam_metric; /* value of ifa_metric */
};
struct ifma_msghdr {
u_short ifmam_msglen; /* to skip over non-understood messages */
u_char ifmam_version; /* future binary compatibility */
u_char ifmam_type; /* message type */
int ifmam_addrs; /* like rtm_addrs */
int ifmam_flags; /* value of ifa_flags */
u_short ifmam_index; /* index for associated ifp */
};
struct if_announcemsghdr {
u_short ifan_msglen; /* to skip over non-understood messages */
u_char ifan_version; /* future binary compatibility */
u_char ifan_type; /* message type */
u_short ifan_index; /* index for associated ifp */
char ifan_name[IFNAMSIZ]; /* if name, e.g. "en0" */
u_short ifan_what; /* what type of announcement */
};
The RTM_IFINFO message uses a if_msghdr header, the RTM_NEWADDR and
RTM_DELADDR messages use a ifa_msghdr header, the RTM_NEWMADDR and
RTM_DELMADDR messages use a ifma_msghdr header, the RTM_IFANNOUNCE
message uses a if_announcemsghdr header, and all other messages use the
rt_msghdr header.
The ``struct rt_metrics'' and the flag bits are as defined in rtentry(9).
Specifiers for metric values in rmx_locks and rtm_inits are:
#define RTV_MTU 0x1 /* init or lock _mtu */
#define RTV_HOPCOUNT 0x2 /* init or lock _hopcount */
#define RTV_EXPIRE 0x4 /* init or lock _expire */
#define RTV_RPIPE 0x8 /* init or lock _recvpipe */
#define RTV_SPIPE 0x10 /* init or lock _sendpipe */
#define RTV_SSTHRESH 0x20 /* init or lock _ssthresh */
#define RTV_RTT 0x40 /* init or lock _rtt */
#define RTV_RTTVAR 0x80 /* init or lock _rttvar */
#define RTV_WEIGHT 0x100 /* init or lock _weight */
Specifiers for which addresses are present in the messages are:
#define RTA_DST 0x1 /* destination sockaddr present */
#define RTA_GATEWAY 0x2 /* gateway sockaddr present */
#define RTA_NETMASK 0x4 /* netmask sockaddr present */
#define RTA_GENMASK 0x8 /* cloning mask sockaddr present - unused */
#define RTA_IFP 0x10 /* interface name sockaddr present */
#define RTA_IFA 0x20 /* interface addr sockaddr present */
#define RTA_AUTHOR 0x40 /* sockaddr for author of redirect */
#define RTA_BRD 0x80 /* for NEWADDR, broadcast or p-p dest addr */

SEEALSO

sysctl(3), route(8), rtentry(9)
The constants for the rtm_flags field are documented in the manual page
for the route(8) utility.